The present invention relates to streamlining construction parts which face a flow of air produced for example by propellers, fans or the like.
Generally speaking it is of advantage to provide propulsion units such as air propellers or fans behind the object to be moved by the propulsion such as a fuselage or even just air foils fans or the like. The specific advantage of this arrangement (rather than placing the aerodynamic propulsion unit in front of these parts) is to be seen in that a certain suction obtains, causing the flow and flow pattern to straighten so as to permit formation of fairly long and coherent laminar boundary layers. in addition, these parts, which are situated in the oncoming flow, will, therefore, not be positioned in an area of rather high jet flow speeds and remain, thus, free from the influence of turbulence, eddies or the like, as they usually occur downstream from an aerodynamic propulsion unit. This explains the higher efficiency of propulsion units when operating by way of "pushing" rather than "pulling".
Static reasons, however, provide constraints on parts to be placed upstream from the aerodynamic propulsion unit, and depending upon the shape and dimension, a certain downstream flow deformation obtaines which increases with higher speeds, and flows off downstream in the plane of the propeller. This locally limited zone has a lower speed in the surrounding free uncoming flow which causes a jump in the effective angle of attack on penetration by the propeller blade and, therefore, produces rotational noise which is a well known phenomenon. Owing to this jump in the angle of attack, moreover, lift as well as drag jump accordingly. This may lead to undesired vibrations in the propeller and is detrimental on the long duration strength of the blades. Also, their tensioning as well as any connecting elements for adjusting the blades likewise experience these vibrations in a detrimental fashion, at least as long duration effects are concerned.
Aircraft which are expected to reach high speeds, and which, therefore, require a correspondingly high angle of attack of the rotor blades, may, moreover, produce a separation of the flow in that downstream flow pattern deformation. That, of course, leads to particularly high loads on the propeller blades. Another consequence of high aircraft speeds is a noticable increase in the rotational resistance of the propeller, because per revolution a braking effect obtains which is proportional to the product of the number of blades and of the number of interfering bodies in the flow path. This, of course, degrades the forward propulsion effectiveness.
Any counter measures for these problems and developed thus far for tending to overcome the drawbacks outlined above, have lead, for example, to an increase in the distance between the trailing edge of the particular "obstacle" and the rotational plane of the propeller. Also, the propeller blades have been shaped particularly in order to avoid one or the other of the drawbacks outlined above, and one has, for example, attempted to use sickle-shaped arrowhead-like propeller blades so that the downstream flow pattern deformations will at least no longer be the entire length of the propeller but is, so to speak, sectionalized and, therefore, these sections are penetrated one after another. This is, for example, disclosed and described in German Pat. No. 26 36 056. The configuration of the propellers, and particularly of shrouded propellers in the manner described in that patent, however, is limited for reasons of the statics involved. This is particularly the case when the propeller is expected to have to take up a high load and is adjustable.